Thermus aquaticus, a bacterial thermophileSource: Govt of Canada
An extremophile is an organism adapted to unusual limits of one or more abiotic factors in the environment. Some of the extreme conditions are temperature, pH, high salinity, high levels of radiation and high pressure. Note that some of these factors, such as temperature and pH, have two extrema. Most extremophiles are micro-organisms such as bacteria and archaea, since higher organisms generally are less adaptive to wide variations from the norm in environmental conditions. Upper limits of existence for carbon based lifeforms appear to be about 150 degrees Celsius, based upon inherent thermal stabilities of amino acids and polypeptides essential to DNA manufacture. In some cases extremophile metabolism thrives on the exotic variation in environmental conditions; in other situations, there is adaptive behavior such as metabolic diapause (cryptobiosis) or desiccation (endosphore formation).

Extremophiles are thought to have been some of the earliest lifeforms on earth, since such early organisms would have to be adapted to harsh conditions, at least in comparison to present day environments. Extremophiles may exist on other bodies in the solar system, such as Jupiter's moon Europa or on Mars. Some microorganism extremophiles have shown industrial potential, such as the ability to remove sulfur compounds from crude oil at high temperatures.

Example environments

There are a variety of natural environments on Earth that have very specialized abiotic factors. These habitats can be in deep marine settings, earth crust interiors, highly mineralized soils, extreme radiation surroundings or extreme aquatic chemical environments.

Hydrothermic deep-sea vents

Microoorganisms and even some marine fauna are often found in around deep-sea ocean floor vents, where water temperatures commonly exceed 80 degrees Celsius (C). In many cases these vents not only host a hyperthermal environment, but also one that is rich in gases such as methane and sulfur compounds, which are actually metabolized by some of the organisms in this marine microclimate.

Thermal deep-sea black smokers

Atlantic Ocean black smoker. Source: NOAA
As a variant to hydrothermic ocean floor vents, there are numerous discharges from marine petroleum deposits that are rich in hydrocarbons and also exhibit elevated thermal regimes. An example microorganism occurring in such a vent chimney is Pyrolobus fumarii, which can metabolize at temperatures as high as 113 degrees C, but cannot thrive in temperatures lower than 90 degrees C. Reproduction of this species is so high that it can double its biomass in 24 hours.

High altitude ice sheets

Combination of extremely low temperatures and low atmospheric pressure are hallmarks of certain high altitude environments such as the Himalaya Mountains. Certain microorganisms and some fauna such as Water-bears can survive in these extreme settings, which are reminders that we are still living in the final throes of the Fifth Ice Age of the Earth.

Permafrosts and methane clathrates

Lower temperature levels for many permafrost and other extreme cold regimes harbor microorganisms that have not been well explored, with little systematic metabolism being at temperatures near or below zero degrees Celsius . Lake Volstok ice cores have produced information that some sulfur reducing archaea are present in the deep methane clathrates.

Volcanic vents

There are numerous locations throughout the world where high temperature steam exudes from volcanic systems, yielding the combination of high temperatures and normally toxic gases. Vents often contain carbon monoxide, which is generally not useful and often toxic to most lifeforms. However, certain microorganisms can metabolize carbon monoxide (CO) at these vent outlets, and some are obligate consumers of the CO. It is important to note that many of the vent systems are extensively separated from the main volcanic vent; for example, in northern Costa Rica, there are hundreds of micro-vents distributed over an area of hundreds of square kilometers, most occurring in rather typical microclimates of the region.

Hot springs

Uzon Caldera, Kamchatka, Russia. Source: Dan Miller/USGS
Extremely high temperatures are created in surface geyser pools, mud pools as well as subsurface aquifers. Many of these environments are habitatable for microorganism thermophiles. Example occurrences of these springs include northern Kyushu, Japan; Yellowstone National Park, USA; Lassen Volcanic National Park, USA; Eureka Hot Springs, Arkansas, USA; Rincon de la Vieja National Park, Costa Rica; San Miguel, Azores; Hveragerdi, Iceland; Gilroy Yamato Hot Springs, California, USA; Uzon Caldera, Kamchatka Peninsula, Russia; Rotokawa region near Taup?, New Zealand. Many of these locales not only are invested with thermophilic bacteria, but also host specialists that are sulfur and hydrogen reducing species, such as Sulfurihydrogenibium rodmanii at Hveragerdi. Some hot springs like those at Taup? and Lassen are notable, because some are highly acidic in addition to their high temperatures.

Salt pans

There are numerous salt pans throughout the world that provide hypersaline environments inhospitable to most lifeforms except for certain salt loving microorganisms and brine shrimp. Many of these environments such as the Makgadikgadi and Etosha Pans in Southern Africa are seasonal wetlands.

Extreme pH waters

Unusually acidic or basic water environments are usually not hospitable to eukaryotes; however, many micro-organisms are adapted to these settings. For example numerous archaea utilize low pH habitats, the most extreme examples being Picrophilus torridus and Picrophilus oshimae, both of which metabolize optimally at pH 0.7. Bacteria are less tolerant of such extreme acidity, but some bacterial species, such as thermoanaeorbacteriumaotearoensis tolerate pH as low as 3.8, along with an associated temperature of 60 degrees C.

Bacteria extremophiles

Deinococcus radiodurans tetrad, resistant to ionizingradiation. Source: Oak Ridge National Lab
Ancestors of extant bacteria are thought to be unicellular organisms, possibly the first lifeforms of the planet Earth, appearing approximately 3.5 billion years before present. Some of the earliest evidence of bacteria is in Onverwacht shales of South Africa. Scientists regard the emergence of bacteria as one of the most astounding events in Earth history, since there are no other proto-lifeforms of their complexity that previously were known to exist on Earth. How and where bacteria developed is one of the great mysteries of science. Bacterial micro-organisms likely diverged from archaea around about 3.2 to 2.5 billion years before present, with their common ancestor possibly a hyperthermophile, e.g. a creature capable of surviving unusually high thermal regimes.

One early bacterium, Kakabekia umbellata, has been determined to have existed in disparate locales including North Wales, Alaska and Iceland; the fact that this set of occurrences all share soils that are ammonia rich has prompted speculation that this bacterium derived from an era when the Earth's environment was rich in ammonia. Until approximately one billion years before present, in fact, Earth's lifeforms were dominated by microscopic creatures such as bacteria, viruses and archaea.

Many bacterial species are extremophiles, which are adapted variously to extreme temperature, pressure, pH, salinity and other abiotic factors. The family Thermaceae is comprised by a number of thermophilic genera, including Thermus, Meiothermus, Marinithermus, Vulcanithermus, Oceanithermus and Truepera.

Bacterial members of genus Deinococcus are highly resistent to extreme doses of ionising radiation. In fact, some bacterial species within each phylum are some type of extremophile, and in some cases a given bacterium may be an extremophile with respect to two or more abiotic parameters.

Archaea extremophiles

Dead Sea genus Halobacteria has halophilicand radiation resistant properties. Source: NASA
Early researchers thought that most Archaeans were extremophiles, existing at the environmental limits of abiotic factor ranges; in recent years, it has been found that numerous Archaea inhabit a broad range of habitats and environmental conditions.

Some Haloarchaea undergo phenotypic switching and grow as several different cell types, including thick-walled structures that are highly resistant to osmotic shock; these thickened walls permit survival hyposaline (low salt) circumstances, but these alternative phenotypes are not actual reproductive structures, but rather may assist the archaea in reaching new habitats.

Indeed, some archaea thrive in extreme temperatures, often above 100 degrees C; for example, they occur in geysers, black smokers, and oil wells. Other viable environments include very cold environments and highly saline, acidic, or alkaline media. For example, Picrophilus torridus, an extreme archaean acidophile, thrives at pH of essentially zero, equivalent to a 1.2 molar concentration of sulfuric acid. Archaea are also found in very cold ocean environments including polar seas. Many archaea also occur throughout the world's oceans among plankton communities as part of the picoplankton. Moreover, archaea include mesophiles that grow in mild conditions, in marshes, sewage, the oceans, and soils.

Halophiles, including the archaea genus Halobacterium, survive in hypersaline environments such as salt lakes and can outcompete bacterial counterparts at salinities greater than 20 percent. Thermophiles grow best at temperatures above 45 degrees C, in locales such as hot springs; hyperthermophilic archaea grow optimally at temperatures greater than 80 degrees C. Strain 166 of the archaean Methanopyrus kandleri Strain 116 survivess at 122 degrees C, the highest recorded temperature for any organism.

Fauna extremophiles

The extremophile Water-bear Hypsibius dujardini.Source: Bob Goldstein
It is much less common for eukyrotes or any multicellular organisms to tolerate extreme environments, although many plant species can survive extremes of soil pH and mineralization. Eukyarotes in general rarely survive temperatures in excess of 60 degrees C.

The most amazing animal extremophiles are certain members of the phylum Tardigrada, or Water-bears, many of which attain a length of 1.5 millimeters. Some species have survived space vacuum conditions (imposing extreme dehydration) and solar/galactic cosmic radiation. Tardigrada belong to a set of anhydrobiotic organisms which have evolved adaptations to survive almost complete desiccation allowing them to withstand the unfiltered solar radiation in outer space. In addition some Tardigrada have been shown to survive temperatures in excess of 150 degrees C, making them the most extreme hyperthermophile on Earth. Conversely some Water-bears have been shown to suvive the extreme cold of one degree Kelvin. Pressure regimes survived by some Water-bears exceed 5000 atmopheres, or higher than that of the deepest ocean trench.

One of the most abundant and widespread groups of arthropod extremophiles are certain brine shrimp, which are tolerant of very high salinities, and in many cases temperature and desiccation extremes.

Alvinella pompejana, commonly called the Pompeii worm is a deep ocean polychaete species occurring solely around hydrothermal vents in the Pacific Ocean, first discovered in the early 1980s near the Galápagos Islands. This amazing creature can survive temperatures of 80 degrees C, classifying it as a hyperthermophile. More amazingly, while its tail is immersed in that elevated temperature, its feather-like head protrudes into cooler waters of 22 degrees C, with its whole 13 cm body being enshrined in a papery tube.

Echinoamoeba therarum is a small thermophilic amoebid organism that has been shown to live in hot springs with temperatures as high as 50 degrees C.

Flora extremophiles

The extremophile Salt cress.
Numerous plant species have tolerance to extreme temperatures, pH, chemical soil concentrations and salinity. Desert environments have produced many examples of plants that are adapted to high temperatures, as well as aridity. Saline environments, such as marsh fringes and salt pans have produced many halophyte species, both upland and submerged vegetation. An example species with polyextremophile dimensions is Salt cress, Thellungiella halophila, a minute annual crucifer; this plant is stable down to -15 degrees C and is tolerant of very high salinity levels.

An important class of extremophiles in North American botany are serpentine endemics. Serpentine refers to a particularly harsh soil type. These plants have a variety of tolerances to chemical imbalance relative to typical ranges of soil metals. These ultramaficRock that is elevated in iron and magnesium content, and low in potassium or serpentine soils are are high in magnesium and low in calcium and also characteristically high in nickel and chromium content; such soils are also typically nutrient poor. Example plants inhabiting such habitats include Leather oak (Quercus durata), Cedars buckwheat (Eriogonum cedrorum) and Tiburon jewelflower (Streptanthus niger). Many of the serpentine restricted species are rare plants, due to the highly specialized environment. Examples of such habitats that are correspondingly rich in rare and endangered endemics are Ring Mountain, Marin County and The Cedars, Sonoma County, both in California.